Early systems

The earliest known system of classifying forms of life comes from the Greek philosopher Aristotle, who classified animals based on their means of transportation (air, land, or water).

In 1172 Ibn Rushd (Averroes), who was a judge (Qaadi) in Seville, translated and abridged Aristotle's book de Anima (On Animals) into Arabic. His original commentary is now lost, but its translation into Latin by Michael Scot survives.

The next major advance in developing scientific classification was made by the Swiss professor, Conrad von Gesner (1516–1565). Gesner's work was a critical compilation of life known at the time.

The exploration of parts of the New World next brought to hand descriptions and specimens of many novel forms of animal life. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. Advances in using this knowledge to classify living beings bear a debt to the research of medical anatomists, such as Fabricius (1537–1619), Petrus Severinus (1580–1656), William Harvey (1578–1657), and Edward Tyson (1649–1708). Advances in classification due to the work of entomologists and the first microscopists is due to the research of people like Marcello Malpighi (1628–1694), Jan Swammerdam (1637–1680), and Robert Hooke (1635–1702).

John Ray (1627–1705) was an English naturalist who published important works on plants, animals, and natural theology. The approach he took to the classification of plants in his Historia Plantarum was an important step towards modern taxonomy. Ray rejected the system of dichotomous division by which species were classified according to a pre-conceived, either/or type system, and instead classified plants according to similarities and differences that emerged from observation.

Linnaean taxonomy

Two years after John Ray's death Carolus Linnaeus (1707–1778) was born. His great work, the Systema Naturae, ran through twelve editions during his lifetime (1st ed. 1735). He is best known for his introduction of a method of modern classification; he created systematic zoology and botany in their present form.

Linnaeus adopted Ray's conception of species, but he made the concept a practical reality by insisting that every species must have a unique Latin binomen, that is, a double name — the first half to be the name of the genus, common to several species, and the second half to be a single word, which is called the specific epithet. This convention is now referred to as binomial nomenclature, and the name formed from the two parts is known as the scientific name or "systematic name" of a species. When a species is further subdivided, the trinomial nomenclature is used. For a scientific name to be correctly written, the genus name must be capitalized, the species epithet must be in lower case and an author label and publication details have to be added.

Before Linnaeus, long many-worded names had been used, sometimes with one additional adjective, sometimes with another, so that no true names were fixed and accepted. Linnaeus' system made it easy to identify unambiguously any given species of plant or animal. He proceeded further to introduce into his system a series of groups: genus, order, class.

The Linnaeus System works by placing each organism into a layered hierarchy of groups. Each group at a given layer is composed of a set of groups from the layer directly below. Simply knowing the two-part scientific name makes it possible to determine the other six layers.

The groupings (taxa) of taxonomy from most general to most specific are:

Several acronym mnemonics have been made for these, for instance King Phillip came over for good soup.

Intermediate ranks may be created by adding prefixes, for instance:

Class

Subclass

Superorder

Order

Suborder

Infraorder

etc.

In addition, species are often subdivided into subspecies and other infraspecific categories (see subspecies, variety, subvariety and form). In certain circumstances prefixes beyond sub- need be used. Some other ranks are also sometimes added. For instance, domains or empires may be given above the level of kingdom, tribes between the levels of family and genus, and sections and series between the genus and species.

Modern developments

The general approach Linnaeus took to classifying species and many of his taxonomic groups have remained standard in biology for at least two centuries. It is now generally accepted that classification should reflect the Darwinian principle of common descent, so that taxa include a single section of the evolutionary tree. Such groups are called monophyletic groups (see also paraphyletic and polyphyletic).

Since the 1960s a trend called cladistic taxonomy or cladism has emerged as a rival to more traditional phylogenetic classification. In this approach taxa are identified with clades, i.e., they can only be monophyletic. In these approaches, the ranking system in Linnaean taxonomy is often not used. A new formal code of nomenclature, the PhyloCode, is currently under development, but many of its rules are in conflict with established codes of nomenclature (both for plants and animals), and it is unclear how the different codes will coexist.

Note in this last example, that most of the taxa are named after the type genus, Magnolia. Sweetbay Magnolia is the first species of the genus named, and is the type species of the genus Magnolia, and thus also of all flowering plants.

Group suffixes

Taxa above the genus level are often given names derived from the type genus. The suffixes used to form these names depend on the kingdom, and sometimes the phylum and class, as set out in the table below.